minerals Article Thermal Alteration of Organic Matter in the Contact of a Rift-Related Basaltic Dyke: An Example from the Black Limestone, Wadi Matulla, West Central Sinai, Egypt Ahmed S. A. A. Abu Sharib 1,*, Ali Q. Selim 2, Mohamed M. Abdel Fattah 1, Safiya M. Hassan 1 and Ioan V. Sanislav 3 1 Geology Department, Faculty of Science, Beni-Suef University, Beni-Suef 62521, Egypt; [email protected] (M.M.A.F.); safi[email protected] (S.M.H.) 2 Faculty of Earth Sciences, Beni-Suef University, Beni-Suef 62521, Egypt; [email protected] 3 College of Science and Engineering, James Cook University, Townsville, QLD 4811, Australia; [email protected] * Correspondence: [email protected]; Tel.: +20-82-1003278107 Received: 16 March 2019; Accepted: 26 April 2019; Published: 6 May 2019 Abstract: In the Wadi Matulla area, central Sinai, Egypt, an asymmetric baked zone having an average width of 103 m was formed on both sides of a sub-aerial rift-related Oligocene basaltic dyke cross-cutting organic matter-bearing chalky limestone of the Upper Cretaceous Sudr Formation. Advection was the significant heat transfer mechanism. Very narrow metamorphic and metasomatic zones are developed in the country rock at the immediate contact with the dyke. The change in the thermal maturation of organic matter is reflected in the differences in values of the total organic carbon (TOC) within the baked zone. Such differences account for the color variation of the snow-white limestone from shades of brown, in the mature to barren samples, to black, in the totally carbonized overmature metamorphic ones. This study presents for the first time the thermal effect of mafic dykes on some exposed organic matter-bearing rocks in the Gulf of Suez (GOS) region, and turns attention to the local maturation of source rocks in contact with rift-related intrusives at a relatively greater burial depth in the rift basin. Keywords: baked zone; organic matter; carbonization; maturation; source rock; thermal alteration; Wadi Matulla; Sinai; Egypt 1. Introduction Transformation of organic matter into oil, gas, or graphite depends upon the conditions of temperature and pressure to which the organic matter-bearing sediments/rocks are subjected, the burial rate of the sediments, and the composition and type of the organic matter [1–4]. Among these factors, the time–temperature burial history of the organic matter-bearing sediments/rocks is of prime importance. Slow rate of heating for a long duration at a convenient temperature (approximately 60–225 ◦C) favors source rock maturation to produce oil and/or gas [1]. On the other hand, conditions of fast rate of heating and/or excessive temperature (approximately >300 ◦C), such as the heat emanating from a nearby magmatic body, may lead directly to the carbonization and/or graphitization of the organic matter [5,6]. The depth of intrusion, water content, and composition of the magmatic body, and the petrophysical properties such as porosity, permeability, and thermal conductivity of the country rock play a significant role in controlling the degree of organic matter transformation. Minerals 2019, 9, 279; doi:10.3390/min9050279 www.mdpi.com/journal/minerals Minerals 2019, 9, 279 2 of 26 TheMinerals Oligo-Miocene 2018, 8, x FOR PEER Gulf REVIEW of Suez (GOS) Rift (Figure1), Egypt, is associated with widespread2 of 27 volcanic activityThe Oligo-Miocene in the form Gulf of maficof Suez dykes (GOS) and Rift sills (Figure intruding 1), Egypt, mainly is associated pre-rift, with and widespread less commonly syn-rift,volcanic successions activity togetherin the form with of mafic a few dykes flows and [7– 9sills]. In intruding the Wadi mainly Matulla pre-rift, area, and the less eastern commonly side of the GOS,syn-rift, a rift-related successions basaltic together dyke intrudeswith a few into flows the [7–9]. Upper In the Cretaceous Wadi Matulla fossiliferous area, the chalkyeastern limestoneside of the beds of theGOS, Sudr a Formation rift-related (Figurebasaltic2 dyke). An intrudes asymmetric into the thermally Upper Cretaceous a ffected zone fossiliferous is developed chalky on limestone both sides of the dykebeds whereof the Sudr the snow-whiteFormation (Figure chalky 2). limestoneAn asymmetric turns thermally into shades affected of greyzone oris developed black. The on thermally both affectedsides chalky of the limestone dyke where has the variable snow-white total chalky organic limestone carbon (TOC)turns into contents, shades some of grey of which,or black. according The to Peters’sthermally classification affected chalky of source limeston rocke has (1986), variable nominate total organic the carbon limestone (TOC) to contents, have a goodsome of to which, very good sourceaccording rock potential to Peters’s [10 ].classification of source rock (1986), nominate the limestone to have a good to very good source rock potential [10]. FigureFigure 1. A 1. general A general structural structural map map showing showing thethe majormajor faults faults and and the the dip dip polarity polarity along along the Gulf the Gulfof of SuezSuez (GOS) (GOS) Rift. Rift. The The general general stratal stratal dip dip direction direction in the northern, northern, southern, southern, and and central central provinces provinces changechange from from SW SW to NEto NE and and back back to to SW, SW, respectively, respectively, across the the Galala-Abu Galala-Abu Zenima Zenima and andMorgan Morgan accommodationaccommodation zones zones (modified (modified after after Bosworth, Bosworth, 2015 2015 [11 [11]).]). Location Location ofof thethe studystudy area is is shown shown by by the smallthe blue small rectangle blue rectangle labeled labeled Figure Figure2. 2. Minerals 2019, 9, 279 3 of 26 Minerals 2018, 8, x FOR PEER REVIEW 3 of 27 FigureFigure 2. 2.(a ()a A) A Google Google Earth Earth mapmap showingshowing the location of of the the study study area area (inset (inset labeled labeled Figure Figure 2 in2 in FigureFigure1), 1), and and the the distribution distribution of of some some Tertiary Tertiary basaltbasalt inin andand outside the the study study area area (marked (marked by by red red stars).stars). (b ()b A) A close-up close-up view view of of the the studystudy areaarea (inset(inset (b) (b) in in (a)). (a)). (c (c) )A A geologic geologic map map of of (b) (b showing) showing the the extensionextension of of the the NNW-trending NNW-trending dyke, dyke, the the major major faults, faults and, and the the di differentfferent rock rock units units cropping cropping out out in in the the study area. Red circles refer to the number and location of the collected samples. Fm., Formation. study area. Red circles refer to the number and location of the collected samples. Fm., Formation. TheThe intriguing intriguing topic topic that that addresses addresses thethe role of igneous activity activity in in the the maturation maturation of ofthe the country country rocksrocks in in producing producing an an extractable extractable quantityquantity ofof hydrocarbonshydrocarbons with with economic economic potential potential has has been, been, and and still is, the subject of abundant literature for decades [12–36]. Analogue models of rift-related igneous still is, the subject of abundant literature for decades [12–36]. Analogue models of rift-related igneous activity and its impact on the formation of hydrocarbon deposits at the local scale paved the way and activity and its impact on the formation of hydrocarbon deposits at the local scale paved the way were the motive for the present study, which presents, for the first time, the thermal effect of a rift- and were the motive for the present study, which presents, for the first time, the thermal effect of a related dyke on rocks that possess source rock potentials in the Gulf of Suez region. rift-related dyke on rocks that possess source rock potentials in the Gulf of Suez region. The purpose of this research is the following: (1) study the thermal effect of the basaltic dyke on theThe Upper purpose Cretaceous of this chalky research limestone is to: (1) beds study of the Sudr thermal Formation, effect of (2) the measure basaltic the dyke change on the in Upperthe Cretaceoustotal organic chalky carbon limestone (TOC) in beds the ofthermally the Sudr affected Formation, zone on (2) both measure sidesthe of the change dyke, in (3) the demonstrate total organic carbonthe color (TOC) change in the in thermally the thermally affected affected zone zone, on both and sides unravel of the the dyke, reason (3) beyond demonstrate the coloration the color in change the inbaked the thermally zone, and aff ected(4) shed zone, light and on unravelthe local the maturati reasonon beyond potential the of coloration source rock in the prone baked formations zone, and in (4) shedthe lightcontact on of the sills local and maturation dykes at a potentialrelatively ofgreater source burial rock depth prone in formations the Gulf of in Suez the region. contact of sills and dykes at a relatively greater burial depth in the Gulf of Suez region. 2. Geologic Setting Minerals 2019, 9, 279 4 of 26 2. Geologic Setting The Gulf of Suez (GOS) is an elongated 300 km long intra-continental Neogene rift basin that represents the extension of the NW–SE-trending Red Sea rift system. The rifting initiated in the Late Oligocene to Miocene times due to the northeast movement of the Arabian plate relative to the African plate [37–39]. The GOS displays the classical rift geometry that is delineated on both margins by extensional fault systems that define classic half-graben style and rotated fault-blocks [37,40–43]. The master and subsidiary faults link up in a characteristic zigzag shape, which resulted from the interaction between the NNW-, N- and NNE-trending fault segments.